The present invention concerns methods and apparatus for the non-invasive monitoring of blood glucose levels by spectrographic analysis of the aqueous humor in the anterior chamber of the eye.
Non-invasive measurement of blood glucose by any method including optical spectroscopy techniques has remained an elusive target for at least two decades. Blood, tissue, and most excreted fluids contain numerous substances which confound glucose spectral signatures. On the other hand, aqueous humor (AH) filling the anterior chamber of the eye (between the lens and cornea) contains relatively few molecules capable of interfering with the spectroscopic detection of glucose. These are primarily lactate, ascorbate, and urea. This fact, and its optically accessible location behind the cornea, make AH an attractive choice as a site on which to attempt non-invasive analysis of glucose.
Pohjola (Acta Ophthalmologica Suppl. 88, 1-80 (1996)) showed that the ratio of aqueous glucose to plasma glucose in normal euglycemic individuals is related to age and ranges from 0.6 to 0.9. He further showed in seven humans with steady-state hyperglycemia that similar ratios applied. No data exists on the equilibration time of aqueous humor glucose with changes in plasma glucose in humans.
Numerous investigators over the years have suggested that the ratio of aqueous glucose to plasma glucose in the normoglycemic rabbit ranges from 0.42 to 1.01 (S. Pohjola, supra; D. Reddy and V. Kinsey, Arch. Ophthalmol. 63, 715-720 (1960); M. Reim et al., Ophthalmologica 154, 39-50 (1967); W. March et al., Diabetes Care 5, 259 (1982)). It is uncertain whether this variability is normal or could be attributed to differences in glucose measurement techniques, collection techniques, sample storage, and anesthesia. The relationship of aqueous glucose to rising, or falling, plasma glucose has not been previously studied in rabbits.
Cotxc3xa9 has reviewed the relative strengths and weaknesses of optical glucose sensing techniques (J. Clin. Engineering 22, 253 (1997)). Raman spectroscopy is potentially attractive because it can distinguish glucose in water solutions containing various levels of other optically active metabolites (S. Wang et al., Applied Optics 32, 925 (1993)). Raman spectroscopy measures the shift in the wavelength of incident light as it is scattered by molecules. Any given molecule causes a characteristic shift in the spectrum of scattered light, which is dependent upon its intermolecular and intramolecular bonds. This is in contradistinction to fluorescence, which is caused by changes in electron energy states, and does not shift relative to the wavelength of incident light.
Wicksted et al, (Appl. Sectroscop. 49, 987 (1995)) suggest that the Raman signature for glucose can be identified in aqueous humor samples, and Goetz, et al (IEEE Trans. Biomed. Eng. 42, 728 (1995)), have demonstrated that higher than physiologic levels of glucose can be measured with Raman spectroscopy in water solutions. J. Lambert et al., (LEOS Newsletter 12, 19-22 (1998)) suggest that measurement of glucose at physiologic levels is possible in water solutions containing other analytes normally found in aqueous humor. When solutions containing fluorescent substances are studied, however, the fluorescence signal can overwhelm the relatively weak Raman-shifted signal. This is a potential problem if Raman spectroscopy is applied to aqueous humor, which contains proteins that fluoresce.
U.S. Pat. No. 5,243,983 to Tarr et al. suggest a non-invasive blood glucose measurement system using stimulated Raman spectroscopy. Stimulated Raman spectroscopy requires the use of both a pump and a probe laser beam. The probe laser beam is used to measure the stimulated Raman light at a single wavelength after transmission across the anterior chamber of the eye. This is undesirable, since an optical component contacting the eye is required to direct the beam across the anterior chamber. In addition, use of a single wavelength may limit the ability to measure glucose at physiologic levels within tissue containing many other Raman scattering chemicals.
U.S. Pat. No. 5,433,197 to Stark suggests a non-invasive glucose measurement apparatus that employs broad band infrared light stimulation.
U.S. Pat. No. 5,553,617 to Barkenhagen suggests a non-invasive method for measuring body chemistry from the eye of a subject by measuring a spectral response such as a Raman scattering response. While it is suggested that the invention may be used for medical applications such as the determination of sugar in diabetics, specific details on how this might be accurately carried out are not provided.
U.S. Pat. No. 5,710,30 to Essenpreis suggests a method for measuring the concentration of glucose in a biological sample such as the eye (see FIG. 4 therein) with interferometric measurement procedures.
U.S. Pat. No. 5,666,956 to Buchert et al. suggests that an instrument for the non-invasive measurement of a body analyte can be based on naturally emitted infrared radiation.
In spite of the foregoing efforts, a commercially viable, non-invasive, blood glucose monitor based on a non-invasive analysis of the aqueous humor of the eye has not yet been developed. Difficulties in developing such a device include correlation of aqueous humor glucose levels to blood glucose levels, the difficulty of obtaining accurate measurements, and the need to minimize damaging effects to the eye caused by excessive exposure to light in an instrument that will be used by subjects on a repeated basis. Accordingly, there is a continued need for new methods for the non-invasive analysis of blood glucose levels.
A first aspect of the present invention is a non-invasive method for determining blood level of an analyte of interst, such as glucose. The method comprises:
generating an excitation laser beam (e.g., at a wavelength of 700 to 900 nanometers);
focusing the excitation laser beam into the anterior chamber of an eye of the subject so that aqueous humor in the anterior chamber is illuminated;
detecting (preferably confocally detecting) a Raman spectrum from the illuminated aqueous humor; and then
determining the blood glucose level (or the level of another analyte of interest) for the subject from the Raman spectrum. Preferably, the detecting step is followed by the step of subtracting a confounding fluorescence spectrum from the Raman spectrum to produce a difference spectrum; and determining the blood level of the analyte of interest for the subject from that difference spectrum, preferably using linear or nonlinear multivariate analysis such as partial least squares or artificial neural network algorithms.
A second aspect of the present invention is an apparatus for the non-invasive determination of the blood level of an analyte of interest such as glucose in a subject. The apparatus comprises:
a laser for generating an excitation laser beam (e.g., at a wavelength of from 700 to 900 nanometers);
an optical system (e.g., a confocal optical system) operatively associated with said laser for focusing the excitation laser beam into the anterior chamber of an eye of the subject so that aqueous humor in the anterior chamber is illuminated;
a detector operatively associated with the optical system and configured to detect a Raman spectrum from the illuminated aqueous humor;
preferably a subtraction system, hardware and/or software processor or other suitable means for subtracting a fluorescence spectrum for said aqueous humor from said Raman spectrum to produce a difference spectrum; and
a processor for determining the blood level of the analyte of interest for said subject from the Raman spectrum (or preferably the difference spectrum). Numerous additional features may be incorporated into the apparatus. The apparatus may include an visual display screen for visually displaying the results of the test to the subject through the same aperture as which the test is conducted. It may include a visual fixation device, also visible through the test aperture, which controls movement of the eye and simultaneously insures that focusing of the laser beam is properly directed into the anterior chamber of the eye. The processor may contain an empirical model of actual testing experience to determine the blood level of the analyte of interest. The apparatus may employ a tunable laser, a plurality of fixed wavelength lasers, or other means for sliding the Raman spectrum passed a plurality of different wavelength detectors to obviate the need for a full grating based Raman spectrometer. A communication line connected to the processor for transmitting the blood level of the analyte of interest to a remote location.
Still other features that can be included in the methods and apparatus described above set forth below.